Thousands of demonstrators were stopped from marching through the streets of Hamburg on Thursday evening after police blocked their protest at the Group of 20 summit in Germany’s second-largest city.

The police said on Twitter that they had temporarily halted the “Welcome to Hell” demonstration from moving forward because some 1,000 protesters were wearing masks to cover their faces and others had shouted anti-police slogans. The police said they were working to arrest people with masks.

The situation in the St. Pauli district, a bastion of left-wing activity for decades, remained tense with authorities telling protesters over loudspeakers to “remove the masks,” and demonstrators responding: “All of Hamburg hates the police.” With the heightened security, traffic in the city has also ground to a halt.

“This is an unprecedented challenge,” Andy Grote, interior minister for Hamburg’s regional government, told reporters earlier in the day. “The police force is prepared. We do have quite a bit of experience with militant activists.”

Hamburg’s 1.7 million residents have largely vacated the center, with legions of armored police -- more than 15,000 drawn from across the country -- fanned out across the port city. Some businesses closed their doors and a number of store owners tried to protect themselves by displaying anti-G-20 messages in their windows, with the hope they’ll be spared in the event of a riot. German Interior Minister Thomas de Maiziere estimates that the number of protesters prepared to use violence stands at about 8,000.

As leaders arrived for the gathering hosted by German Chancellor Angela Merkel that begins on Friday, protest organizers are pledging “various and unpredictable mass resistance” as they make their way close to the summit venue this evening.

The “Welcome to Hell” organizers denounce the G-20 as a vehicle for global capitalism, “an assembly of bourgeois governments, torture states and warring military blocks reinforcing global exploitation,” according to the group’s manifesto. The march will include a “large organized black bloc,” including anarchists, the group said.

Police said some 30 protest events have been organized. A group called Block G-20 pledges to disrupt the meeting by using diversionary tactics against police, everything from marches to dancing and street theater. Primary targets for disruption are the summit venue, Hamburg’s City Hall and the Elbphilharmonie concert hall.

Another organization said it will seek to shut down commercial activity in Hamburg’s port, an export hub for Europe’s largest economy that itself is reliant on global trade.

Protests will culminate on Saturday with a demonstration that organizers hope will draw as many as 100,000. It will begin at 11 am at Hamburg’s main train station.